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- Battery Know-what/why (Article 4)
This week’s article focuses on the cycle metrics of lithium-ion batteries (LIBs). Some LIB cell performance characteristics fluctuate within a single charge/discharge cycle. What do you do whenever your phone battery runs out of juice (the end of a discharge cycle)? You connect it to the power source thereby initiating a charge cycle. The changes in a cell’s characteristics during discharge necessitate the subsequent charge cycle and therefore a reversal of the aforementioned characteristics. Some of those ‘cycle‘ characteristics are state of charge (SOC), energy efficiency and self-discharge.
State of charge (SoC)
Know-what: this is a cycle metric that expresses the instantaneous capacity (level of charge) of a battery cell with respect to its capacity when fully charged. State of charge is expressed as a percentage of the battery cell’s capacity at full charge with 0% indicating a fully drained battery and 100% indicating a fully charged battery. The battery icon on your phone screen displays the phone battery cell’s SoC.
Analogy: imagine a water tank - SoC is analogous to the instantaneous actual volume of water in a water tank expressed as a percentage of the water tank’s total volume.
Know-why: usage duration - SoC dictates when to start or stop charge or discharge cycles.
Depth of discharge (DoD)
Know-what: quantifies the decrease in SoC during the discharge cycle. DoD is expressed in percentage.
Know-why: indicates how much battery capacity has been consumed.
Self-discharge
Know-what: refers to the loss of charge when a battery cell is at rest. This is normally characterized by a drop in SoC when the cell is not in use. This phenomena arises from parasitic electrochemical reactions in the cell.
Analogy: imagine a leaking water tank - the level of water in the tank falls even though water is not being consumed for a useful purpose. Similarly, for a battery cell, leaking charge does not perform useful electrical work.
Know-why: dictates how long a battery can hold stored charge. Efforts are made to minimize self-discharge.
Charge efficiency
Know-what: refers to the ratio of the energy stored in a battery cell at the end of a charge cycle to the energy consumed by the cell from a power source during the charge cycle. The value is always less than 100%.
Know-why: indicates a cell’s energy losses during the charge cycle.
Discharge efficiency
Know-what: refers to the ratio of the energy released by a battery cell during discharge to the energy stored by the cell prior to the beginning of the discharge cycle. The value is always less than 100%.
Know-why: indicates a cell’s energy losses during the discharge cycle.
Roundtrip efficiency (RTE)
Know-what: refers to the ratio of the energy released by a battery during discharge to the energy consumed by the battery from a power source during charge. The value is a combination of the charge and discharge efficiencies.
Know-why: indicates a battery cell’s total energy losses during a complete charge and discharge cycle.
Energy efficiency